Means for obtaining even illumination from a single source



p 1954 E. GRETENER 2,689,298

I MEANS FOR OBTAINING EVEN ILLUMINATION FROM A SINGLE SOURCE Filed Sept. 1, 1948 2 Sheets-Sheet l EDGAR GRETENER affo megs 2,689,298 MEANS FOR OBTAINING EVEN ILLUMINATION FROM A SINGLE SOURCE Filed Sept. 1, 1948 E. GRETENER Sept. 14, 1954 2 Sheets-Sheet 2 INVENTOR Edgar Grez ezzer ATTORNEYJ Patented Sept. 14, 1954 MEANS FOR OBTAINING EVEN ILLUMINA- TION FROM A SINGLE SOURCE Edgar Gretener, Zurich, Switzerland Application September 1, 1948, Serial No. 47,314

10 Claims.

The present invention relates to illumination and particularly concernsthe even illumination of a space of substantial dimensions by a single light source, Reference is made to my British Patent No. 664,250 cognate with the present application. The invention comprises an optical device for Obtaining uniform illumination of a given space of predetermined dimensions by means of a single light source, characterized by the of a light reflecting or refracting surface in conjunction with the light source and in which said surface reflects or refracts the light rays emanating from said source in such a way, that all reflected or refracted rays disposed in a common vertical plane intersect at or near one common point in front of the space to be illuminated, whereas all reflected or refracted light rays disposed in a common horizonal or approximately horizontal plane intersect at or near one common point at the reariof said space to be illuminated. I

The fundamental idea of the invention is that homogeneous illumination of a selected space may be obtained if this space is illuminated by a light beam, the sectional area of which is approximately of equal size all through this space. As the same light flux passes through each sectional area and is uniformly distributed over each of these areas and as the intensity of iilumination is equal to the quotient of the light flux divided by the sectional area of the beam, intensity of illumination will thus be equal for any sectional area. A light beam having this property is obtained by causing the beam to converge in horizontal sections and diverge in vertical sections. This divergence and convergence is achieved by a deflector device having a form such that all rays reflected by points lying on common horizontal sections through said device unite in common points, respectively, in front of the device, and that the elongations of all rays reflected by such points as lie in common vertical sections through said device unite in common points, respectively, to the rear of said device. The uniting points in front of the device, the real images, are located on a practically perpendicular line and the uniting points to the rear of said device, the virtual images, are located on a horizontal line rearward of the surface. The space to be illuminated is located in that part of this light beam where convergence and divergence just compensate each other, thus keeping the sectional area constant. This means that the space to be illuminated must be located in between th reflector surface and the 2 real image of the light source. The area to be illuminated must be interposed into the beam of reflected rays between the said device and the locus of said uniting points in front of said device.

These, objects may be achieved only if the reflector surface as a whole is given a toroidal shape, 1. e., convex in vertical sections and concave in horizontal sections, or is provided with Fresnel-lenses or prisms of equal effect.

It is an object of the invention to provide an arrangement to illuminate a selected area evenly from a concentrated light source by employing a light spreading area which forms a real image of the light source in horizontal planes by main rays uniting in points behind the space to be illuminated and also forms a virtual image of the same light source in vertical planes by main rays uniting in points in front of the space to be illuminated.

The even illumination of a space in length, breadth and height such as a stage presents a problem of illumination technique which is of especial practical importance in the illumination of studio sets or other scenes for motion picture photography, television and the like. Evenness of lighting is important not only in color work but also in black and white.

The use of a single light source to obtain the necessary illuminationis most desirable so as to minimize the trouble and expense of operation and maintenance and maximize control for given equipment. We are here concerned with even illumination commonly referred to as the keylight of a stage, studio, or scene which should not only have a homogeneous distribution, but is preferably white light having a continuous spectrum. Additional lighting is obtained from other light sources such as spot-lights with or without colored filters to produce any special effects which may be desired. This special lighting is not to be confused with key lighting.

The homogeneous distribution of light through out a space is now obtained in practice by using either a distant source of light to reduce the contrast caused by the operation of the inverse square law or a light source which extends over a relatively large area.

According to the present invention a light spreading surface which may be either a reflector or refractor or combination of the two is providedwith suitably shaped optical surfaces constructed insuch a manner that image of the light source is formedbehind the space being illuminated by all rays reflected by points of said surface lying in a common horizontal plane and a virtual image of the light source is formed in front of said space by all rays reflected by points of said surface lying in a common vertical plane.

The light source may consist, for example, of a single arc lamp or an aggregation of such lamps which are combined to form a single source of light. It is preferred, in the practice of the invention, that a cylindrical rather than a point source of light is formed. An elongate fan shaped beam is best suited for the illumination of the reflecting, refracting or combination light spreading surface. The disclosure in my copending U. S. Patent No. 2,621,284, filed July is, 1%7, for carbon arcs, may be employed as a suitable cylindrical source.

The accompanying drawings are intended to be taken as illustrative of three forms the invention may take and are not to be taken as limiting.

ike numerals refer to like elements throughout.

Fig. 1A is a sectional plan View of a schematic construction using a reflector incorporating the invention.

Fig. 1B is a sectional elevation of Fig. 1A.

Fig. 2A is a sectional plan view of a schematic construction using an echelon lens of the type developed by Fresnel, incorporating the invention.

Fig. 2B is a sectional elevation of a vertical Fresnel lens embodying the invention.

Fig. 3 is a fragmentary elevation of a lenticulated lens according to the invention.

Fig. 4 is a fragmentary section of one of the lens elements of Fig. 3 with an elemental ray diagram.

Fig. '5 is a sectional perspective View of a Fresnel type lens embodying the invention.

Fig. 6 is a vertical section of the structure of Fig.5.

Referring to Figs. 1A and 1B, ll indicates a cylindrical light source having its longitudinal axis vertical and is preferably of the arc-lamp type mentioned above. A light spreading optical device If comprisin a doubly curved reflecting surface made up of a portion of a toroid surface having different radii of curvature about sections normal to each other, specifically the horizontal and vertical sections. The curvature in horizontal sections is concave as is shown in Fig. 1A and the curvature in vertical sections is convex as is shown in Fig. 1B. The resulting toroid surface presents a shape similar to that of a saddle. The structure of element 12 may be formed of metal such as aluminum, copper or the like, polished or plated so as to have high reflecting characteristics. Elements l2 may also be made of glass on which a layer of metallic silver has been deposited. Coated plastics can be used.

The space under illumination is taken to be a cube having a length l3, a height [4 and a breadth l5 representing a set in a motion picture film studio, the stage of the theatre or a set to be televised. The last perhaps has the most exacting illumination requirements.

The main rays Iii and I? in elevation View Fig. 1B and. I8 and ii) in plan view Fig. 1A from light source H are reflected and reoriented by light spreading surface l2 to form individual well-defined polar diagrams It the central rays of which are represented by the corresponding reflected beams l6 and I1 in the vertical plane and i8 and it in the horizontal plane. As the ray analysis is independent of the particular outline of polar diagrams IE] it is sufficiently accurate in the interests of simplicity to consider only main rays 16, I7, I03 and i9 coming from light source H and deviated by reflector 52 as rays l6, ll, l8 and It.

The practical homogeneous or even distribution of light within the space l3l l--|5 requires that the following conditions be fulfilled:

First, the main rays l6 and ll coming from light source ll toward reflector [2 in vertical planes be deviated by reflector l2 so that a virtual image ll' of source If is formed by the reflected main rays I6 and ll in front of the space !3! ll5 as shown at I! in Fig. 13.

Second, the main rays l8 and I9 coming from light source il toward reflector ii. in horizontal planes be deviated by reflector I2 so that a real image ll of the light source H is formed by rays l8 and I9 behind space l3-Hl-l5 as shown at i I" in Fig. 1A.

Third, light intercepting objects Within the space l3-M-l5 should be small compared with the dimensions of the space. Should a single solid light impervious object fill a half or twothirds of space I3M-l5 there would probably be shadows depending on the contour of the object which would interfere to a greater or less degree with the even distribution of light in space I3--Ml5.

Fourth, the brightness of reflecting surface I2 must be substantially a constant over the area of the surface. This requires that the reflection characteristic over the surface l2 and the brightness level over th surface of light source ii are constants or that they compensate each other, whereby the resultant reflected light level in space |3-I 4-! 5 is made constant.

The even distribution of light within the space l3, l4, I5 is obtained by reason of the fact that the luminous area of the surface increases proportionally with the distance of the view point from the said surface provided that the brightness of the surface is made constant.

For practical purposes the bringing of rays I6, I1 and l8, l9, and all other rays lying in the same vertical or horizontal plane, respectively, to a sharp focus is unnecessary as only a limited concentration is needed to produce the effect desired and the soft lighting effect of relatively large areas of diffusion may in some cases be desirable. In other words image points H and l I" need not be sharp.

For practical purposes the distribution of illumination over the spreading surface is advantageously made homogeneous in every vertical section through the light source and the spread surface. 1

Surface It may be white such as coated plastic. When it is desired that polar diagrams ill be of any special shape, reflector l2 as shown by Figs. 3 and 4 is composed of a large number of individual light spreading elements 68 such as miniature concave or convex mirrors or reflectors formed in or on the material from which the light spreading reflector I2 is made. These miniature reflectors preferably form a regular pattern. The curvature of the individual elements is chosen in such a way, that the light impinging upon the reflecting surface of an individual element or in the direction of arrows $2 is reflected to fill a given solid angle 63 with a prescribed distribution of intensity, which may be indicated by the length of arrows 64, the points of which form the distribution curve. In a particular case as shown in Fig. 4, light is reflected with homogeneous intensity into a certain angle 65, whereas no light is reflected beyond the angle. Where the individual reflectors are very small and form a deposited mosaic regularity of pattern is of less importance and may be neglected.

Figs. 2A and 2B show a form of invention in which the light spreadingdevice 50 comprises two crossed echelon lens systems of the Fresnel type. Fig. 2B shows an elevation of a horizontal Fresnel lens grid system 30 forming one part of lens 50 and Fig. 2A shows a plan view of a vertical Fresnel lens grid system 40 completing lens 50.

In Fig. 2B Fresnel lens grid system 30 forms a virtual image I I of light source H in front of space lt--l-'l-l5 as shown by the ray construction l6, l1 andrefracted rays l6, H.

In Fig. 2A Fresnel lens grid system forms a real image I I" behind space Balk-l 5as shown by the ray construction l8, l8 and refracted rays l8, l8.

For clarity, the horizontal lenticulations 30 of Fig. 23 have been omitted from Fig. 2A and vice versa. The embodiment illustrated in Figs. 2A and 21? has the disadvantage that it occupies more space which in. devices of this type isat a premium.

In most cases of practical application the reflector type I2 is preferred particularly where the illuminating equipment is to be portable.

A described above the light spreadin device such as reflector I 2 comprises a toroid shaped reflecting surface having different radii of ourvature in the vertical and horizontal sections, the vertical extent being of considerably smaller dimension than the horizontal one as is seen in Fig. 1B and Fig. 1A respectively. For certain applications a single curved surface combined with a Fresnel mirror or prism grid system ma be used as shown by Figs. 5 and 6. i

Fig. 5 schematically shows in a perspective view an embodiment of the invention and Fig. 6 a section view thereof. Elements corresponding to elements shown in Figs. 1A and 1B are given identical reference numerals. The light coming from a source ll impinges upon a refractor 10 which is cylindrical i. e. concave in horizontal sections and straight in vertical sections. The reflecting surface 1| supports circular Fresnel prisms l2. Light coming from source I I traverses th prisms i2, is reflected by the underlying surface ll, traverses the prisms a second time and is thus reflected downwards to greater or lesser extent, as the deflecting power of the Fresnel prisms increases from the top towards the bottom. It is to be understood that in realty the dimension of the prisms are much smaller and have been enlarged in the drawings only for the sake of clarity. The rays impinging upon the reflector are deflected in such a way that all rays, e. g. iii-l'd-TE or l6-11l8, reflected by points on the reflector that are disposed in common horizontal section thereof converge due to the curvature of the reflector surface H towards a common point respectively, 19 or 80. All such points are disposed on line II. All rays reflected by pcints that are disposed in common vertical sections, e. g. l3l6 or l4--'|1 or -18, due to the effect of the Fresnel prisms diverge, so that their rearward elongations intersect in a common point, respectively e. g. BI, 82 or 83, rearwards of the reflector. All such points are disposed cn a lin H. The entire light beam, defined in the drawin by rays 13, 15, 16 and 18, diverges in one e. g. the vertical direction, and converges in the other, e. g. the horizontal direction. The space l3--l4--l5 to be illuminated is interposedinto this beam in between reflector ll and the locus of intersection of the reflected rays, i. e., the locus of the real images of the light source. With this construction a virtual light source image is formed in front of the space l3l4l5 by the horizontal Fresnel mirror system by all rays reflected by points of said surface lying on a common vertical section thereof. respectively, and another real image. is. formed by all rays reflected by points of said surface lying on a common vertical section thereof, respectively behind the space by the cylindrical reflecting surface. The. term front as here used refers to the left side of space l3l l'-l5 in the drawing.

The Fresnel lens 50 may be made translucent or transparent depending upon the softness of key lighting required. Lens 50. may. be a fiat spread surface, the focusing being done by the lenticular elements.

Iclaim:

1. An illumination system comprising a compact light source, a single unbroken light spreadin surface having two fixed radii of curvature about two different axes in respect to a relatively large solid region to be illuminated, the curvature of said surface about one axis producing a real image of said source on one side of said region and removed from said surface a distance of at least a meter, the curvature of said surface with respect to the other axis beingsuch as to produce a virtual image of said sourceon another side of said region whereby said region is evenly and homogeneously illuminated.

2; An optical system comprising a compact cylindrical light source, a light spreading surface of a toroid shape, the curvature in a substantially vertical section of the surface producing a virtual image of said source and the curvature in a substantially horizontal section of the surface producing a real image of said source, the 1ongitudinal axis of said cylindrical source being substantially vertical with respect to the curvatures of said surface, said images being on different sides of said surface whereby even illumination is produced over a selected area of at least a square meter in extent interposed between said surface and the locus of said real images.

3. The combination set forth in claim 1, said light spreading surface being a reflector having a concave curvature about one axis thereof and a convex curvature about another axis.

4. The combination set forth in claim 1, said light spreading surface comprising an echelon lens having elemental lens portions on opposite sides thereof.

5. The combination set forth in claim 1, said light spreading surface comprising a curved reflector with a Fresnel mirror.

6. The combination set forth in claim 5, said light spreading surface comprising a reflecting surface curved in the horizontal plane and a Fresnel mirror whereby the real image is formed by said reflecting surface and the virtual image is formed by said Fresnel mirror.

7. An illuminating system for the production of homogeneous illumination of a region, said system comprising a compact cylindrical light source, a single light spreading reflecting surface illuminated by said source, every horizontal section of said surface being an unbroken concave configuration and causing the light rays from said source to converge in horizontal planes, every vertical section of said surface being an unbroken convex configuration and causing the light rays from said source to diverge in vertical planes, whereby said region is illuminated in effect by two images of said source, said cylindrical light source being centrally located with respect to the area of said reflecting surface and said source having its longitudinal axis substantially vertical with respect to the said surface.

8. An illumination system for illuminating a solid area with an even distribution of light, comprising a compact cylindrical light source facing the forward surface of a reflector device, said reflector device deflecting the light rays emanating from said source and comprising a concave configuration in all horizontal sections so that all rays reflected by points lying on common horizontal sections through said device unite in common points, respectively, before said forward surface, said deflector device comprising a convex configuration in all vertical sections so that the elongations of all rays reflected by such points as lie on common vertical sections through said device unite in common points, respectively, behind said forward surface, said area to be illuminated being interposed into the beam of reflected rays in a space measuring at least a meter between the forward surface of said device-and the locus of said uniting points before said forward surface, the longitudinal axis of said cylindrical light source being substantially vertical with respect to the said horizontal and vertical sections of said reflector device.

9. The combination set forth in claim 8, the radius of curvature of said reflector in said vertical sections being substantially smaller than the radius of curvature in said horizontal sections said light source comprising an arc-lamp device centrally positioned before said forward surface.

10. An optical system for providin substantially uniform illumination of a selected region of substantial macroscopic dimensions, a light source, a light deflecting device having as a whole a toroidal shape effectively convex in vertical sections and concave in horizontal sections thereof, a single illuminating beam from said source for illuminating said region, said beam being convergent in horizontal sections and divergent in vertical sections, the rays of said beam affected by points lying on common horizontal sections of said device, uniting in common points, respectively, in front of said device to form real images removed from said device a distance of not less than the order of a meter, said selected region bein positioned between said device and said real images in the area in which the horizontally convergent sections of the single illuminating beam and the vertically divergent sections of the beam mutually compensate to produce substantially uniform illumination.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 755,447 Cahills Mar. 22, 1904 1,279, 62 Clark Sept. 17, 1918 FOREIGN PATENTS Number Country Date 745,364 France Feb. 13, 1933 

